If $x \XBox y=(x+y)^2$ for positive integers $x$ and $y$, then $4 \XBox 5=(4+5)^2=9^2=81$. What is the value of $2 \XBox 3$?
Since $x \XBox y=(x+y)^2$ for positive integers $x$ and $y$, $2 \XBox 3=(2+3)^2$. We have $$(2+3)^2=5^2=\boxed{25}.$$